Air-cooled heat exchangers such as fluid coolers and condensers reject heat to the atmosphere. These devices reject heat by sensible heating of the ambient air; therefore the lowest temperature they can achieve is some temperature above the ambient dry bulb temperature. By use of adiabatic cooling, the ambient air can be cooled to a temperature approaching the wet bulb temperature. This pre-cooled air is then used to reject heat. By use of adiabatic cooling, a dry-cooling heat exchanger can be made smaller (less expensive) or can cool to a lower temperature (more energy efficient) or some combination of the two.
There are two typical ways that adiabatic cooling is performed. One way is to cool the air with saturated pads. Thick pads are placed at the inlet to the air-cooled heat exchanger. These pads are saturated with water. When incoming air is drawn across these pads, some of the water is evaporated and the air is cooled. Although these pads are in widespread use, they have several drawbacks. To full saturate the pads, a heavy stream of water needs to be run over the pads. Most of this water is not evaporated and is either sent to drain or recirculated. Sending this water to drain is very inefficient, while recirculation requires another system to treat and periodically drain the water. Additionally, these pads are made of a material that absorbs water and they have a life expectancy of only a few years before needing to be replaced. Furthermore, the pads are left in place year round, even when adiabatic cooling is not used. The pads cause a resistance to air flow and require higher fan horsepower all year round.
The second typical way to generate adiabatic cooling is by the use of misting nozzles. Misting nozzles generate small droplets of water that quickly evaporate thus cooling the air. Misting nozzles spray water at a lower rate than water is streamed over the saturated pads, thus there is no need for a recirculation system and less water is used. The nozzles do not cause any resistance to air flow, so fan horsepower is kept at a minimum. One issue with misting nozzles is that the minerals that are contained in the spray must pass through the coils and these minerals can cause issues. In a pad system these minerals stay with the excess water that is sent over the pads or is trapped on the pads themselves.
To prevent scaling, particularly of calcium carbonate, soft water or softened water must be used with misting nozzles. If hard water is sprayed, scale can form at the nozzles and on the coils. To minimize this problem many manufacturers severely limit the number of hours that the adiabatic sprays can be run each year. Scaling can be avoided by using softened water. Softening replaces the +2 valance cations in the water with sodium. Sodium salts are highly soluble and thus will not form a scale. The concern with softened water is that all of the anions that were present in the hard water are still present in the softened waters. These anions, particularly chloride, sulfate, and hydroxide, can be very corrosive to the coils and fins. This is particularly true if the salts are allowed to stay on the coils for extended period of time. To minimize these corrosion effects many manufacturers limit the number of hours that the adiabatic sprays can be run each year with softened water.
The solution for running extended hours with an adiabatic spray system is to use very low mineral water. Typically reverse osmosis (“RO”) water is used for these extended-hour systems. Low-cost RO systems are available that can provide sufficient RO water to operate a cell at a reasonable cost. These low-cost units operate off of domestic water pressure without the need of a separate high-pressure pump. These RO devices should be fed softened water for best membrane life. The RO will remove most of the sodium ions as well as most of the corrosive anions. The resulting water is often less corrosive than rainwater to the materials of construction of the heat exchanger.
There are issues with using these low-cost RO systems for adiabatic cooling. One is that these systems are inefficient on water use. The table below illustrates the output of a low-cost, high-volume RO. Fully 65% of the raw softened water is discarded in order to generate 35% clean water.
AlkalinitySampleSodiumChlorideSulfate(hydroxide)% of FlowInput120 ppm57 ppm24 ppm168 ppm100%RawSoftenedFeed-WaterOutput 2.5 ppm 1 ppm>1 ppm 5 ppm35%RO PermeateWaterOutput183 ppm85 ppm41 ppm247 ppm65%RO RejectWater
Another issue is that even though a single unit is not too expensive, a single unit can provide sufficient misting for only about a single cell; most units will have 4 or more cells thus requiring multiple RO units.